One of the major unanswered questions in the study of leukemogenesis concerns the identity of the normal lymphohemopoietic precursor cells that are transformed by various murine leukemia viruses. In this proposal, we plan to define the cellular origins of the thymus-dependent leukemias that are induced in rats by the rat-adapted Gross leukemia virus (RAGV). Our working hypothesis, based on previous studies of RAGV-induced leukemias, is that the leukemogenic targets of the RAGV are terminal deoxynucleotidyl transferase-positive (TdT+) prothymocytes or their immediate precursors (pre-TdT+ cells). We will use two approaches to test this hypothesis. In the first, we will assess the ability of the RAGV to induce leukemic transformation in vivo in purified populations of lymphohemopoietic stem and progenitor cells isolated from rat bone marrow on the fluorescence-activated cell sorter (FACS). In the second, we will study the ability of TdT+ and pre-TdT+ bone marrow cells, generated in a selective culture system, to be transformed in vitro by the RAGV. The leukemogenic potential of the virally transformed cells from both systems will be determined by adoptive transfer assays, using the RT-7 alloantigenic pan-T cell marker to determine the origin of the leukemic T cells that arise in histocompatible LEW/NBR strain chimeras. The cellular targets of both the in vivo and in vitro transformation experiments will be characterized and compared with each other and with their preleukemic and leukemic descendants. The efficiency of leukemic transformation and viral infection within a defined target cell population will be determined, using quantitative in vitro leukemic colony-forming unit and focus-forming unit assays, respectively. The time course of appearance and the relative numbers of preleukemic and leukemic cells will be traced, as will any progression in their differentiation and leukemogenic potential. The possible role of thymus epithelial cells in enhancing the leukemogenic process and influencing the phenotype of the leukemic cells will also be investigated. The results of these studies should permit the leukemogenic process initiated by the RAGV to be traced from the level of the normal precursor cell to the preleukemic cell to the overtly leukemic cell. Moreover, the results will serve as a reference for comparative studies of the leukemogenic targets of other MULVs; and may provide important insights into the probable cellular origins of similar TdT+ lymphoblastic leukemias and lymphomas that occur in human patients.